XNATMAP Website Mapping with Australian Map Coordinates or Data

Introduction

In 2020, Australia adopted a new Geocentric Datum of Australia (GDA2020). The adoption of GDA2020 was, among other things, to eliminate the nearly 30 years of tectonic plate motion. Recognised plate movement had introduced an increasing divergence in position, of some 7 centimetres per year to now total around 1.8 metres, from the coordinates based on the 1994 Geocentric Datum of Australia (GDA94).

The last national Australian map coverage was the Geoscience Australia 1: 250 000 scale series of 516 maps produced between 1995 and 2012 based on GDA94. At this scale the 1.8 metre divergence represents a negligible 0.007 millimetre difference in coordinates on the map. For XNATMAP historical research, however, national Australian map coverage and coordinates produced on the 1966 Australian Geodetic Datum (AGD66) and the earlier R502 map series, are of interest. Under the birdcage of satellite orbits now used for establishing GPS position on the Earth’s surface the divergence, if any, from these earlier datums and that of today, needs to be known. Furthermore, any limitations with integrating such information in Web Mapping applications, must be recognised.

Australia’s National Map Coverages

Australia’s first national map coverage was the mainly uncontoured R502 series. This map series comprised 540 printed map sheets at a scale of 1: 250 000 using a transverse Mercator projection and a corresponding yard grid based on the Clarke 1858 datum. The yard grid was used for the R502 map series until replaced in 1966 when the metric Australian Map Grid (AMG) based on AGD66, was adopted. After adoption of the AMG, almost half the R502 map sheets were overprinted with the 10 000 metre AMG in a cyan colour. The last R502 map sheet went to press in 1968.

National Mapping Director, Bruce Phillip Lambert in his 1956 paper, The National Geodetic and Topographic Survey, stated that the simplest and most economical method of establishing horizontal control was to make astronomical observations for latitude and longitude at a sufficient number of points and treat the observed values as absolute geodetic coordinates on some particular reference ellipsoid. The R502 map series which predominately relied on astronomical star observations or astrofixes, for controlling horizontal position, adopted the Clarke 1858 reference ellipsoid. The Australian origin for the Clarke 1858 datum was determined in 1933 as the Sydney Observatory when the R502’s specifications were established.

Around 1965, the National Topographic Map Series (NTMS) was initiated to gradually replace the R502 series. This contoured map series at 1: 250 000 scale was produced on a Universal Transverse Mercator projection in metres, based on the AGD66 datum. The NTMS nationwide coverage of 516 fully contoured printed topographic maps was completed in 1988.

Follow on programs revised and reprojected the NTMS data on the Map Grid of Australia (MGA) based on the GDA94 datum. These maps and data are available today with a stated currency of 1995 to 2012.

Coordinates extracted from any of the above mapping published before 1994 will not be on the GDA94 datum and will thus differ from their later GDA94 and GDA2020 based coordinates.

Behind GPS Coordinates

As simply as possible, the World Geodetic System (WGS), of which the latest revision is WGS84 (G2139), is the datum used by the GPS operated by the USA. WGS84 is a global datum, which means that coordinates change over time for objects which are fixed on the ground.

Since its inception in 1987, WGS84 has been revised six times in 1994 (WGS84 (G730)), 1996 (WGS84 (G873)), 2002 (WGS84 (G1150)), 2012 (WGS84 (G1674)), 2013 (WGS84 (G1762) and 2021 (WGS84 (G2139)). To uniquely identify which WGS84 is being used the alpha-numeric G code where G stands for GPS and the number i.e. 2139 is the GPS week, is used.

WGS84, while being useful as a global datum, due to its history is now considered to have a reduced accuracy in the order of less than 5 metres. Added to this is that it is a static datum in being unable to account for tectonic movement. Historically then hand held GPS positions, gathered in real time and ignoring tectonic movement in WGS84, simply accepted the false assumption that WGS84 then was equal to GDA94. The result now being that such WGS84 GPS coordinates will differ from their GDA94 and GDA2020 coordinates.

Australian Map and Coordinate Datums

Tables 1.6 and 1.7 below are a summary of Australian datums as published in the Intergovernmental Committee on Surveying and Mapping (ICSM), Geocentric Datum of Australia 2020 Technical Manual.

Table 1.6: Summary of the parameters of Australian datums.

Datum	Geographic Coordinate Set	Grid Coordinates	Reference Frame	Ellipsoid / Spheroid	Semi-major axis (m)	Inverse Flattening
GDA2020	GDA2020	MGA2020	ITRF2014	GRS80	6378137.0	298.257222101
GDA94	GDA94	MGA94	ITRF1992	GRS80	6378137.0	298.257222101
AGD84	AGD84	AMG84	-	ANS	6378160.0	298.25
AGD66	AGD66	AMG66	-	ANS	6378160.0	298.25

The ellipsoid recommended by the International Association of Geodesy (IAG) and used with the GDA is the Geodetic Reference System 1980 ellipsoid.

The GRS80 and WGS84 ellipsoids have a very small difference in the inverse flattening (please refer table below), but this difference is insignificant for most practical applications.

Ellipsoid	GRS80	WGS84
Semi major axis (a) metres	6 378 137.0	6 378 137.0
Inverse flattening (1/f)	298.257222101	298.257223563

Table 1.7: EPSG codes of Australian datums.

Datum	Geographic Coordinate Set	EPSG Code Geodetic Datum	EPSG Code Geodetic CRS (Geocentric)	EPSG Code Geodetic CRS (Geographic 3D)	EPSG Code Geodetic CRS (Geographic 2D)
GDA2020	GDA2020	1168	7842	7843	7844
GDA94	GDA94	6283	4938	4939	4283
AGD84	AGD84	6203	-	-	4203
AGD66	AGD66	6202	-	-	4202

Note : The Geomatics Committee of the International Association of Oil and Gas Producers (IOGP), previously the European Petroleum Survey Group (EPSG), maintain a reliable, freely available registry of geodetic and transformation information. ESPG codes now often accompany the description of datums to uniquely identify its parameters.

Establishing Datum Divergence

Accepting the different datums, the extent of any shift between those datums and the datum of today had to be quantified.

The following analysis includes the common Australian mapping datums but is only indicative based on the available coordinates of the Sydney Observatory over time.

The Sydney Observatory was selected, as the comparison point, as it was the origin for the Clarke 1858 datum used for the national R502 map series, established in 1933. Also reliable historical coordinates for the Observatory were available for 1912 and 1865. For simplicity, a spherical Earth with a radius equivalent to that used for the WGS84 and GRS80 ellipsoids of 6 378 137 metres was used for the calculations. This method will indicate the maximum divergence between today’s coordinates and the Observatory’s coordinates on the earlier datums.

With the 2020 coordinates for Sydney Observatory fixed the divergence from the 2020 coordinates back thru time can be seen in the table below. In the earliest years the divergence was essentially due to the uncertainty with longitudes in Australia generally. In the periods of interest, 1933 and later, it is clear from the table that any published positional data earlier than 1994 or extracted from published maps of the R502 or NTMS (AGD66) series will be displaced by some 200 metres (7 seconds of arc) from its post GDA94 or GDA2020 position.   

YEAR	Latitude (DMS)			Longitude (DMS)			Divergence (m)	Direction (DD)	Notes
2020	-33	51	34.47000	151	12	16.47000
1994	-33	51	34.61023	151	12	16.47061	4.3	359.8	Accepted nationally as 1.8 metres due tectonic plate shift
1984	-33	51	40.32410	151	12	12.25570	210.9	30.9
1966	-33	51	40.29240	151	12	12.31360	209.3	30.6	ICSM divergence : 194-207 metres
1933	-33	51	41.10000	151	12	17.85000	208.1	350.1	Specifications for R502 map series
1912	-33	51	41.10000	151	12	23.10000	266.5	320.3	Surveyors General Conference, Melbourne
1865	-33	51	41.10000	151	14	57.90000	1083.4	280.9	First time Observatory listed in Nautical Almanac

GPS coordinates recorded on hand held devices were also considered. The Magellan company is said to have introduced the first hand held GPS device, the NAV1000, in 1989. In March 1990, however, the USA for security reasons introduced what was known as Selective Availability. From 1990 to 2000 this resulted in civilian GPS positioning to be degraded to around ±100 metres. In normal circumstances obtaining GPS coordinates to three or more decimal places of a degree would mean a position to better than 100 metres, but the inbuilt 100 metre degradation was the dominant factor. It is therefore considered that any hand held GPS coordinates obtained before 2000 and thus degraded by 100 metres, would likely show a similar displacement to its GDA94 or GDA2020 position.

As described above, the WGS84 is a dynamic reference frame as at the beginning of each year its coordinates are adjusted, at the half year mark, to account for plate tectonic motion. Thus WGS84 GPS coordinates collected post 2000, will show today some 0.5–2 metres of apparent horizontal motion in comparison to their GDA94 coordinates.

What about if historical positional data is used in Web Mapping Applications?

Web Mapping applications such as Google Maps, Google Earth, Mapbox, Bing Maps, OpenStreetMap, Mapquest, Esri, Virtual Earth, and others use a Mercator projection based on the World Geodetic System (WGS84). To save computing resources, however, such applications project on a sphere rather than an ellipse. This WGS 84 / Pseudo-Mercator or Web Mercator uses the same formulae as the standard Mercator for small-scale maps. However, the Web Mercator uses the spherical formulae at all scales whereas large-scale Mercator maps normally use the ellipsoidal form of the projection. The discrepancy is imperceptible at the global scale but causes maps of local areas to deviate slightly from true ellipsoidal Mercator maps at the same larger scale. The Web Mercator is limited for use between 85.06°S and 85.06°N as the deviation becomes more pronounced the further from the equator.

The WGS 84 / Pseudo-Mercator or Web Mercator is not a recognised geodetic system. Nevertheless, its use has it registered with the unique EPSG code 3857. The radius of the projection sphere is the same as the WGS84 semi major axis being 6 378 137 metres. Due to the misunderstandings in use and deviations inherent in the Web Mercator the United States Department of Defense has declared this map projection to be unacceptable for any official use!

The risk lies in the fact that the Web Mercator’s stated use of WGS84, which is static, means that dynamic WGS84 based coordinates, are not compatible with it. This is most critical for high accuracy, sub-metre, applications.

For Australia, this is best explained in the Intergovernmental Committee on Surveying and Mapping (ICSM), GDA2020 Implementation Working Group (GMIWG), Advisory on WGS84 and Web Mapping of 15 June 2020 :

For several years Australian jurisdictions have been aware of challenges with mixing GDA94 and GDA2020 data through the web mapping medium. The issue originates from early days of web mapping when WGS84 and its common Web Mercator projection were adopted globally as the default or hub datum for web mapping. In 1994, GDA94 and WGS84 were considered equivalent.

Since WGS84 is a dynamic (or time-dependent) datum in which coordinates of features slowly change as a result of ongoing tectonic motion, this equivalence with the static GDA94 slowly degraded until it remained accurate only at the metre level [the degradation with earlier static datums i.e. Clarke 1858, AGD66, being much greater].

When GDA2020 was defined, for most users, the equivalence was restored meaning GDA94 ≈ WGS84 ≈ GDA2020 for low accuracy applications. This equivalence does not hold for higher accuracy (less than 1 metre) applications.

See also the WGS84 and misaligned spatial data in Australia information sheet.

The following table summarises what this all generally means when using published Australian coordinates or extracting them from the published national Australian map series, for use in a Web Mapping application.

Map  Series	Currency	Datum	When used in a Web Mapping Application :
NTMS or later	1995-2012	GDA94	Generally expect reasonable compatibility
NTMS	1965-1988	AGD66	Expect a divergence of some 200 metres
R502	1955-1968	AGD66 overprint	Expect a divergence of some 200 metres
R502	1955-1968	Astrofix/Clarke 1958	Expect a divergence of some 200 metres
Any published coordinates pre 1994 (#)			Sources vary but generally expect a probable divergence
Note (#) : Examples include coordinates on datums such as Bessel, Clarke, WGS72, AGD84 and various Australian State/Colonial/Local datums.

As an example, the AGD84 and GDA94 coordinates for the Sydney Observatory from the above table, were displayed in Google Earth. While the GDA94 coordinates fell within 15 metres of the Observatory, the AGD84 coordinates fell some 200 metres to the south west.

GPS and other historical coordinate data sets have many sources, acquisition strategies and a range of datums. Integration of such historical coordinate data sets with Web Mapping applications will need to be assessed on a case by case basis.

Consequently

Historical mapping data published on the XNATMAP website has and will continue to be managed within the accuracy limitations described above.

In many of the website maps the national context has meant that historical data could be safely used as the 200 metre shift was insignificant at map scale (0.2 millimetres at 1: 1 000 000 map scale and even less at smaller scales).

In other cases the source of the data is or will be quoted and any caveats, or conversions of the data, noted.

by Paul Wise, June 2021